1. Field of the Invention
The present invention relates generally to a dot line printer of the type in which a hammer bank containing a plurality of print hammers is moved reciprocally forward and backward along a print line by a shuttle mechanism.
2. Description of the Related Art
Dot line printers having a hammer bank that is moved reciprocally forward and backward along a print line are well known in the art. The hammer bank contains a plurality of print hammers juxtaposed along the print line and capable of printing a plurality of dot lines simultaneously as the hammer bank makes one scan along the print line from one extreme to another.
In the printers of this type, heat is generated from a hammer coil associated with a print hammer when a printing pattern calls for repeated driving of that print hammer. The heat generated from the hammer coil can reduce the service life of the print hammer parts. To resolve this problem, a circuit constructed as shown in FIG. 1 has been incorporated in the printer to control the printing process.
According to this circuit configuration, a print controller 15 controls the transfer of print data from a print data storage memory 21 to a print buffer 22. Simultaneously, the print data is sent to a counter 16 provided for counting the number of times each print hammer 14 is driven. A timer circuit 19 set to a fixed interval T1 is started at the beginning of the printing process and is restarted after the elapse of each interval T1. Each time the interval T1 elapses, the counter 16 is reset and the timer circuit 19 is restarted.
A comparator 17 compares the number of times each print hammer is driven, as counted by the counter 16, with a reference value that is latched in a data latch circuit 23 and detects when the number of times a print hammer is driven reaches the reference value. The reference value represents an allowable number of times a print hammer can be driven during each interval T1 without shortening the service life of the hammer parts. If the comparator 17 indicates that the number of drives for a print hammer as counted by the counter 16 has reached the reference value during an interval T1 of the timer circuit 19, a print halting unit 18 halts printing from the time of the detection until the current interval T1 of the timer circuit 19 has elapsed.
On the other hand, if the comparator 17 does not indicate that the number of drives for a print hammer, as counted by the counter 16, has reached the reference value, then the print halting unit 18 does not halt printing and printing continues. After the interval T1 of the timer circuit 19 has elapsed, the count set in the counter 16 is initialized to zero for each print hammer. The timer circuit 19 is restarted for another fixed interval T1 and printing continues.
FIG. 2 is an example of a timing chart showing print control according to the process described above.
In the example of FIG. 2, the timer circuit 19 is operated repeatedly for fixed intervals T1. As shown in the diagram, the number of times the print hammer is driven reaches the reference value during the first interval T1. When the comparator 17 detects that the number of drives has reached the reference value, printing is temporarily halted until the next timing interval T1 begins in order to prevent the hammer parts from becoming too hot. In the next time interval T1, the comparator 17 again detects that the number of times the print hammer is driven has reached the reference value, and printing is again halted until the next timing interval T1 begins.
Hence, according to the above-described technology, the drive number for each print hammer is counted during each specified timing interval T1. When the number of drives for a print hammer reaches the reference value, printing is halted until the next timing interval T1 begins in order that the service life of the print hammer will not be shortened by heat generated from the corresponding hammer coil.
However, not all printing patterns use all of the print hammers simultaneously. For example, as shown in FIG. 3, despite the fact that the print hammers can print N dot lines simultaneously as the hammer bank makes one scan from one extreme to another along the print line, only the upper half of the N dot lines are printed and the lower half of the N dot lines are not printed. That is, among the print hammers mounted on the hammer bank, a half of the print hammers for printing the lower half of the N dot lines are not used at all during each scan of the hammer bank whereas the rest of the print hammers are used for printing as indicated by shaded area in the diagram. When printing this type of pattern using the circuit configuration shown in FIG. 1, the printing process will be stopped even before the print hammers have reached the limits of their capability. Hence, printing speed and efficiency is lost.